From: Tejun Heo tj@kernel.org
commit 65de03e251382306a4575b1779c57c87889eee49 upstream.
cgroup writeback tries to refresh the associated wb immediately if the current wb is dead. This is to avoid keeping issuing IOs on the stale wb after memcg - blkcg association has changed (ie. when blkcg got disabled / enabled higher up in the hierarchy).
Unfortunately, the logic gets triggered spuriously on inodes which are associated with dead cgroups. When the logic is triggered on dead cgroups, the attempt fails only after doing quite a bit of work allocating and initializing a new wb.
While c3aab9a0bd91 ("mm/filemap.c: don't initiate writeback if mapping has no dirty pages") alleviated the issue significantly as it now only triggers when the inode has dirty pages. However, the condition can still be triggered before the inode is switched to a different cgroup and the logic simply doesn't make sense.
Skip the immediate switching if the associated memcg is dying.
This is a simplified version of the following two patches:
* https://lore.kernel.org/linux-mm/20190513183053.GA73423@dennisz-mbp/ * http://lkml.kernel.org/r/156355839560.2063.5265687291430814589.stgit@buzz
Cc: Konstantin Khlebnikov khlebnikov@yandex-team.ru Fixes: e8a7abf5a5bd ("writeback: disassociate inodes from dying bdi_writebacks") Acked-by: Dennis Zhou dennis@kernel.org Signed-off-by: Tejun Heo tj@kernel.org Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/fs-writeback.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)
--- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -582,10 +582,13 @@ void wbc_attach_and_unlock_inode(struct spin_unlock(&inode->i_lock);
/* - * A dying wb indicates that the memcg-blkcg mapping has changed - * and a new wb is already serving the memcg. Switch immediately. + * A dying wb indicates that either the blkcg associated with the + * memcg changed or the associated memcg is dying. In the first + * case, a replacement wb should already be available and we should + * refresh the wb immediately. In the second case, trying to + * refresh will keep failing. */ - if (unlikely(wb_dying(wbc->wb))) + if (unlikely(wb_dying(wbc->wb) && !css_is_dying(wbc->wb->memcg_css))) inode_switch_wbs(inode, wbc->wb_id); }